Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a frequent cause of autism and epilepsy. The long term goal of this project is to elucidate the mechanisms underlying FXS as a prerequisite to the development of more effective treatments. FXS results from loss-of-function mutations in the X chromosome gene FMR1 (fragile X mental retardation 1), which encodes FMRP (fragile X mental retardation protein). Studies of FMRP in the past two decades have focused on its function in synaptic development. Recent analyses, however, have revealed a key role for FMRP in neural stem cell (NSC) proliferation in the embryonic Drosophila brain and in the adult mouse brain. Moreover, our preliminary data suggest that FMRP is important for maintaining the NSC pool by regulating actin dynamics during mouse neocortical development. Because abnormalities in NSCs may result in incorrect number and/or type of neurons produced and therefore impaired brain function, investigating the role of FMRP in NSCs will be important for understanding the pathogenesis of FXS. This project will test the hypothesis that FMRP and one of its homologs, FXR1, act redundantly to maintain the NSC pool during neocortical development. In Specific Aim I, knockdown and knockout analyses will be performed to determine how loss of FXR1 affects the NSC pool and actin dynamics during neocortical development. In Specific Aim II, Fmr1 and Fxr1 mutant mice will be crossed to assess potential redundant functions of FMRP and FXR1 in regulating the NSC pool and actin dynamics during neocortical development. Successful completion of this project will establish the role of the FMRP family members in NSCs during neocortical development, and therefore provide basis for thorough analysis of potential developmental NSC defects in FXS human patients in future studies. Developmental NSC defects will represent a new direction in studying the pathogenesis and therapy of FXS. In addition, successful completion of this project will raise the possibility that members of the FMRP family may be novel drug targets for manipulating NSC proliferation in stem cell-based therapies.